This invention relates to electrophoretic displays in general and more particularly to a power supply for properly biasing and maintaining reliable operation of such a display.
The prior art is replete with many references which relate to electrophoretic displays. Reference is made to U.S. Pat. No. 4,655,897 issued on Apr. 7, 1987 to Frank J. DiSanto and Denis A. Krusos, the inventors herein, and assigned to Copytele, Inc., the assignee herein. Essentially, that patent discloses an electrophoretic display apparatus which includes a planar transparent member having disposed on the surface a plurality of vertical conductive lines to form a grid of lines in the Y direction. On top of the grid of vertical lines there is disposed a plurality of horizontal lines in the X direction which are positioned above the vertical lines and insulated therefrom by a thin insulating layer at each of the intersection points.
Spaced above the horizontal and vertical lines patterns is a conductive anode plate. The space between the conductive plate and the X and Y line patterns is filled with an electrophoretic suspension containing chargeable pigment particles. When a voltage is impressed between the X and Y lines, pigment particles which are located in wells or depressions between the X and Y pattern are caused to migrate toward the conductive plate and are deposited in accordance with the bias and drive signals supplied to the X and Y line conductors.
The patent also describes the operation and fabrication of such displays. In any event, there are many other references which pertain to electrophoretic displays.
Basically, as indicated above, the electrophoretic display consists of a suspension of pigment particles dispersed in a dye solvent of contrasting color. The solvent as well as the particles is placed into a cell which basically consists of two parallel and transparent conducting electrodes designated as the anode and cathode. Many such cells in the prior art also employ a grid electrode which further controls the transportation of charged particles. See the above-cited patent for an examples of this type of display.
In operation the charged particles are transported and forced against one electrode as the anode or cathode under the influence of an applied electric field so that the viewer may see a desired pattern formed by pigment particles. When the polarity of the field is reversed, the pigment particles are transported and packed on the opposite electrode. As indicated, the prior art is cognizant of such devices.
A particularly interesting application which is copending herewith is entitled METHOD AND APPARATUS FOR OPERATING AN ELECTROPHORETIC DISPLAY BETWEEN A DISPLAY AND A NON-DISPLAY MODE, filed on July 14, 1986, Ser. No. 885,538, U.S. Pat. No. 4,746,917, for Frank J. DiSanto and Denis A. Krusos, the inventors herein, and assigned to the assignee, Copytele, Inc.
In that application there is shown an electrophoretic display which is operated in a first mode where the display operates to display data and has normal DC voltages applied to its electrodes. During a second mode or a non-display, mode a suitable alternating voltage of a given frequency and magnitude is AC coupled to the anode electrode of the display for a predetermined time interval to cause pigment particles to settle between the anode and cathode whereby the effective life of said display is increased. The transfer of the display mode to the second mode is afforded by suitable switching circuitry.
That application describes the biasing of the various electrodes of the electrophoretic display and particularly describes operation during a non-display mode and a display mode.
As seen in that application, there is shown the various electrodes associated with the electrophoretic display and the biasing of the electrodes in the first and second modes. The patent application shows various relays which are utilized to power the display during the operational mode and to remove power from the display during a non-operating mode.
The electrophoretic display has a particular advantage in that once data is written into the display, the data can remain displayed for extended periods of time without the utilization of any biasing potential.
The electrophoretic display operates in many modes. One mode is the Write Mode. In the Write Mode the anode voltage is positive to allow pigment particles to migrate to the anode under control of signals applied to the grid and cathode. During an Erase Mode, particles which migrated to the anode are removed from the anode thus erasing the display. In the Erase Mode the anode is negative. There is also a Hold Mode. During the Hold Mode, the anode voltage is positive and essentially the Hold Mode is similar to the Write Mode in that the anode is positive and is awaiting the receipt of data. As one will understand, the electrophoretic display is changed during operation from the Hold or Write Mode to the Erase Mode to thereby erase data and then write data back into the display.
This is a typical operation. In order to perform such operations, the anode voltage, as will be explained, is switched between a positive and a negative level indicative of the Write or Hold Mode as compared to the Erase Mode. There are other modes which are associated with the electrophoretic display one of which is indicated in the above-referenced co-pending application. In these modes the anode electrode is supplied with suitable AC operating potentials either coupled to the anode electrode via a capacitor or directly applied to the anode electrode at relatively slow rates. The technique of applying an AC signal to the anode is described in regard to the above-noted co-pending application to prevent agglomeration clustering.
The electrophoretic display has been analogized in operation to that of a vacuum tube triode. Hence, the various electrodes for such a display have been designated as the anode, the grid and the cathode. While the analogy has some basis in regard to understanding operation of the display, an electrophoretic display is subjected to many variations which are not provided in a typical vacuum tube triode. As noted, the electrophoretic display is associated with a suspension of pigment particles dispersed in a dye solvent of contrasting color. This affords the medium through which the particles are directed to the anode or cathode of the display. Due to this medium, the characteristics of the display vary greatly. These variations depend upon the recent history of the display such as when it was last operated and in what mode. Certain of the characteristics of the display depend on the operating temperature as well as how long the display was inactive. Most of these phenomenon are due to the chemical nature of the suspension as well as the characteristics of the solvent and pigment particles.
In particular it is indicated that when the anode electrode of such a display is switched to the Erase Mode or the Hold Mode, the anode current peaks to a value as high as four times the steady state current then falls to a value somewhat below the steady state current and finally settles to the steady state current. In the case of switching the anode to the Hold Mode the high peak current decreases the amount of pigment left in the wells of the display and thereby decreases the brightness of the display. In addition, since the initial data applied to the panel drives all the grids positive and the anode current is higher than steady state at this time, a larger amount of pigment is moved from the wells than would normally be the case if the anode current were at steady state.
The brightness of the panel is therefore reduced. Furthermore, as indicated above, the anode current as a function of anode voltage is dependent upon the recent history of the panel such as when it was last operated and in what mode. In addition, the dip in anode current, below steady state, may cause the initial portion of the image to be lighter than the remaining image which is written when the anode current has reached steady state. Thus, as one can ascertain from the above, the effective impedance of the electrophoretic display varies as a function of these different conditions. This impedance variation of the display effects the display brightness.
It has been found that by utilizing a constant current supply to drive the anode of such a display, many of the above-noted problems are avoided.
It is an object of the present invention to provide optimum operation of electrophoretic displays by biasing the anode of the display with a constant current source.
It is a further object of the present invention to provide an electrophoretic display which is capable of displaying a uniform bright image by biasing said display with a constant current source during the Hold, Write and Erase Modes.